The worldwide use of wireless devices such as cellular telephones is growing at a rapid pace. As the number of people using wireless devices increases, the number of features offered by wireless service providers increasingly matches the features offered by traditional land-line telephone service providers. Features such as call waiting, call forwarding, caller identification (caller I.D.), three-way calling, and others are commonly offered by both land-line and wireless service providers. These features operate in the same manner on both wireless devices and land-line telephones. Enhanced 911 (also known as E911) services, however, operate differently on land-line telephones (normally referred to as a “911” call) than on wireless devices.
When a 911 call is placed from a land-line telephone, the 911 reception center receives the call and determines the origin of the call. In case the caller fails, or forgets, to identify his or her location, the 911 reception center is able to obtain the location from which the call was made from the land-line telephone switching network and send emergency personnel to the location of the call.
If instead, an E911 call is placed from a wireless device such as a cellular telephone, the E911 reception center receives the call but cannot determine the origin of the call. If the caller fails, or forgets, to identify his or her location, the E911 reception center is unable to obtain the location of the call because the mobile switching network is different than the land-line telephone switching network. At present, the best that the E911 reception center may possibly do is determine the location of the base station corresponding to the cell from which the call was placed. Unfortunately, typical cells in a cellular system may cover an area with approximately a 30 mile diameter.
A proposed solution to this problem is to use a wireless positioning system that includes satellites and/or pseudolites (base stations) to triangulate the position of a wireless device. The Global Positioning System “GPS,” also known as NAVSTAR, is an example of a satellite based navigation system that may be used by a wireless device in combination with an appropriate GPS receiver to pinpoint its location on earth. The array of GPS satellites transmits highly accurate, time coded information that permits a receiver to calculate its exact location in terms of latitude and longitude on earth as well as the altitude above sea level. The GPS system is designed to provide a base navigation system with accuracy to within 100 meters for non-military use and greater precision for the military.
The space segment of the GPS system is a constellation of satellites orbiting above the earth that contain transmitters, which send highly accurate timing information to GPS receivers on earth. The fully implemented GPS system consists of 21 main operational satellites plus three active spare satellites. These satellites are arranged in six orbits, each orbit containing three or four satellites. The orbital planes form a 55° angle with the equator. The satellites orbit at a height of 10,898 nautical miles (20,200 kilometers) above earth with orbital periods for each satellite of approximately 12 hours.
Each of the orbiting satellites contains four highly accurate atomic clocks. These provide precision timing pulses used to generate a unique binary code (also known as a pseudo random or pseudo noise (PN) code) that is transmitted to earth. The PN code identifies the specific satellite in the constellation. The satellite also transmits a set of digitally coded ephemeris data that completely defines the precise orbit of the satellite. The ephemeris data indicates where the satellite is at any given time, and its location may be specified in terms of the satellite ground track in precise latitude and longitude measurements. The information in the ephemeris data is coded and transmitted from the satellite providing an accurate indication of the exact position of the satellite above the earth at any given time. A ground control station updates the ephemeris data of the satellite once per day to ensure accuracy.
A GPS receiver configured in a wireless device is designed to pick up signals from three, four, or more satellites simultaneously. The GPS receiver decodes the information and, using the time and ephemeris data, calculates the approximate position of the wireless device. The GPS receiver contains a floating-point processor that performs the necessary calculations and may output a decimal display of latitude and longitude as well as altitude on the handset. Readings from three satellites are necessary for latitude and longitude information. A fourth satellite reading is required in order to compute altitude.
Unfortunately, a problem to this solution is that, oftentimes, one or more of the signal sources (from either the satellites and/or pseudolites) will interfere with other signal sources, due to individual signal strength. In a sense, one signal will overwhelm, or jam, other signals, so that computation of the position of the wireless device is not possible. Therefore, there is a need for a cancellation system that cancels the strongest signals until a suitable configuration of signal sources may be detected for computing the position of the wireless device.